Re: Why forbid "INSERT INTO t () VALUES ();"

This is not the only area where empty tuples are not supported. Consider:

PRIMARY KEY ()

This should mean the table may only contain a single row, but is not supported.

Also, GROUP BY supports grouping by no columns, but not in a systematic way: Using aggregate functions with no explicit GROUP BY clause will result in grouping by no columns (i.e., entire result set is one group); I also found that I could GROUP BY NULL::integer, abusing the column number syntax. But things like GROUP BY ROLLUP () are not supported.

On the plus side, empty rows are supported, although the explicit ROW keyword is required.